The primary structures of 15 procaryotic 5S RNAs are known; however, there is no firm evidence concerning the function of this molecule in protein synthesis or its mode of interaction with the ribosome. One approach used by this laboratory to probe 5S RNA structure-function relationships is by chemical modification. Recently we have found that the 5S RNA can be readily released from the Escherichia coli ribosome by chemical reaction which does not involve a modification of the 5S RNA per se. This finding enables us to study the nature of 5S RNA-ribosome interactions, i.e., how the 5S RNA is anchored to the ribosome. We have also been able to selectively block (up to 50%) one highly crucial region of the 5S RNA, the guanine residue of the invariant sequence 5' C42CGAAC473'. We propose to investigate the ability of the 5S RNA blocked in this invariant sequence to: 1) Bind specific ribosomal proteins, 2) function in protein synthesis when reconstituted in ribosomes. These experiments may provide information on RNA-protein recognition sites as well as the functional role of the 5S RNA in protein synthesis.